1. Field of the Invention
The present invention relates to an active matrix display device which is designed to improve the image quality on the display screen of an active matrix display device.
2. Description of the Related Art
FIG. 2 schematically shows a conventional example of an active matrix display device. The region 204 enclosed by the dashed line in the drawing is a display region, and thin-film transistors 201 are provided in a matrix array in this region. The line connected to the source electrode of a thin-film transistor 201 is an image (data) signal line 206, and the line connected to its gate electrode is a gate (selection) signal line 205.
Considering now the drive element, the thin-film transistor 201 affects data switching and drives a pixel cell 203. A capacitance 202 provides for holding image data in a capacitor. The thin-film transistor 201 provides for switching image data constituted by voltages that are imposed on the pixel. Designating the gate voltage of the thin-film transistor as Vg and its drain current as Id, the relationship of Vg-Id is shown in FIG. 3. That is, when the gate voltage is in the thin-film transistor's off region, Id becomes large. This is called the OFF current.
In the case of an N-channel thin-film transistor, when Vg is negatively biased, the OFF current is determined by the current that flows in a PN junction, which is formed between a P-type layer, which is induced at the surface of the semiconductor thin film and an N-type layer between the source region and the drain region. Since, many traps are present in the semiconductor thin film, this PN junction is imperfect, and the flow of junction leakage current can easily occur. The reason why the OFF current becomes greater with increasing negative bias on the gate electrode is that the carrier concentration in the P-type layer formed at the surface of the semiconductor thin film increases, and the energy barrier at the PN junction becomes narrower. Consequently the field becomes concentrated, and the junction leakage current increases.
The OFF current that arises in this manner is greatly dependent on the source-drain voltage. For example, it is known that the OFF current increases dramatically as the voltage imposed across the source and drain of a thin-film transistor becomes larger. In more detail, the OFF current when a voltage of 10 V is imposed across the source and drain is not double the OFF current that flows when a voltage of 5 V is imposed but instead may be as much as 10 times or even 100 times greater. This nonlinearity is also dependent on the gate voltage. Generally, the difference between the two is considerable when the value of the gate electrode reverse bias is large (a large negative voltage in the case of an N-channel type element).
A circuit diagram of a conventional X shift register is shown in FIG. 4 (A). This X shift register is a circuit which produces gate electrode on/off timing for thin-film transistors that drive the pixel electrodes of an active matrix display device. The output signals of the shift register, which, as is clear from FIG. 4 (A), is constituted by flipflops, are as shown in FIG. 4 (B). ANDing of adjacent signals within these output signals gives a signal plot such as in FIG. 4 (C) with which the thin-film transistors of each row in an active matrix display device are successively brought to an on state.